This invention relates to a low temperature cryoprobe and more particularly to a portable, hand held probe usable within a small confine to produce a point source of gas at a relatively constant temperature of 77 degrees Kelvin.
The metallurgic and semi-conductor industries, among others, use apparatus which must perform in cryogenic temperatures, that is, below 80 degrees Kelvin. Pumps, refrigerators, electrical circuits, electronic circuit boards and other equipment are required to perform within an abnormally cold environment.
As is well known, extreme cold changes the properties of materials, and operators often discover that malfunctions of equipment cannot be duplicated and the causes of malfunctions cannot be isolated when equipment is returned to room temperature. For example, epoxy seals, wire joints, carbon resistors, germanium resistance thermometers, and thermocouple junctions may open only during a cooling cycle of operation.
The terms "cryoprobe" and "cryogenic probe" have been used in the prior art to describe a particular type of instrument suited to the needs of the medical industry. The instrument is used to direct gas at cryogenic temperatures toward human and animal tissue, as an adjunct to what is termed "cryosurgery". (See U.S. Pat. Nos. 4,074,717, Feb. 21, 1978, Schulze; 4,236,518, Dec. 2, 1980, Floyd; 4,376,376, Mar. 15, 1983, Gregory.)
In general, these cryosurgical devices fall within one of two categories, namely, those utilizing high pressure gas to produce low temperatures by the Joules-Thompson phenomena and the second utilizing liquid cryogen. Joules-Thompson equipment typically utilizes gas stored at 500 to 600 psi which is expanded to atmospheric pressure in close proximity to tissue to be necrotized. Such equipment requires adequate safeguards to protect the patient and operator against the hazards associated with these high pressures. Cryogenic devices using liquid cryogen typically operate at relatively low pressures of about one atmosphere and can apply a spray of finely divided liquid cryogen directly to the surface to be necrotized. As cryosurgical devices, the liquid cryogen devices are probably superior, in that they are faster, and an adequate supply of cryogen can be stored in a small heat insulated container readily held and manipulated in the operator's hand.
The present invention combines certain methods known in the prior art with a significant additional feature to provide a low temperature cryoprobe that overcomes shortcomings of the prior art.
As is known in the prior art, material to be cooled is placed in contact with a cold refrigerant near its vaporization temperature. Heat is transferred from the material to be cooled to the refrigerant, warming the refrigerant above its vaporization temperature and forming a gas above the liquid. Changes in the refrigerant's pressure, or rate of flow, or the addition of heat will increase the cooling rate of a given refrigerant. (See U.S. Pat. No. 3,628,347, Apr. 13, 1970, Puckett et al.)
A second known method embodiment in the current invention is the placing of a fluid to be cooled within a helical coil, thereby increasing the contact area between the fluid and the refrigerant. (See U.S. Pat. No. 2,223,152, Jan. 8, 1940, Nagin; U.S. Pat. No. 2,241,186, Mar. 16, 1940, Coons; U.S. Pat. No. 3,696,636, Oct. 10, 1972, Mille.)
At least one cryosurgical probe incorporates the above two methods, as well as the use of multiple nozzles, allowing various densities of flow. (See U.S. Pat. No. 4,116,199, Dec. 6, 1976, Bryne.)
None of the devices described are adequate, however, for the needs of the low temperature researcher. Speaking generally, the low temperature researcher needs an instrument that can be directed at materials which are the suspected cause of a malfunction and that can produce a flow of gas that temporarily returns just the suspect materials to low temperature conditions for the purpose of testing continuity and integrity.
To meet this need, the researcher's instrument must meet specific performance standards. First, and most important, the researcher must be assured that the probe gas is at a constant temperature, eliminating variations in temperature as causes for variation in equipment performance. It is not sufficient that the probe gas exits below 32 degrees fahrenheit, or even zero degrees fahrenheit, as with most cryosurgical probes. It is not sufficient that the temperature of the probe gas was 77 degrees Kelvin when it left the vapor bath if the bath is located remotely, allowing the probe gas to warm substantially by the time it exits the cryoprobe. To meet the low temperature researcher's needs, the temperature of the probe gas when it exits the cryoprobe must be precisely known and controlled.
In addition, the low temperature researcher must be able to use helium as a probe gas in his cryoprobe. Nitrogen is easily sustained at low temperatures and is thus desirable for use in cryosurgical probes. But, nitrogen is the major component of air, making it difficult to detect the presence of nitrogen test gas in a normal laboratory environment. On the other hand, helium introduced to detect leaks in joints and seals will be detectable in small amounts using a helium sensitive mass spectrometer leak detector--an instrument widely used in the industry.
Further, the low temperature researcher must be able to direct a probe gas with precision. Laboratory concerns for low thermal conductivity and minimized heat leak often require that electrical leads be the size of a human hair. The researcher must be able to isolate these leads and other minute details of construction with an equally precise flow.
Finally, the low temperature researcher must be able to sustain the flow of probe gas for extended periods of time, and the cryoprobe must continue to function, despite the fact that the cryoprobe device itself becomes extremely cold.
It is an object of this invention to provide an improved cryoprobe which maintains a flow of cryogen at substantially 77 degrees Kelvin with an accuracy of plus or minus two degrees.
It is another object of this invention to provide an improved cryoprobe which directs a flow of cryogen from a point source, and can therefore be directed with precision.
It is another object of this invention to provide an improved cryoprobe which draws upon a virtually unlimited source of cryogen, thereby insuring operation for a substantially unlimited duration.
It is another object of this invention to provide an improved cryoprobe which uses no mechanical functions, and is not subject, therefore, to breakdown of mechanical functions due to sustained cold.
It is another object of this invention to provide an improved cryoprobe which can use helium gas as its cryogen or probe gas, making it compatible with helium gas detectors commonly found in the industry.
Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.